Most commercial pressurized water reactors (PWRs) have core modules which are axially supported at the bottom by a core support plate and are held down at the top by springs. Thus, the modules are subjected to column-type compression loading. Since it is desired that the fuel assemblies contain a minimum of structural material in order to minimize parasitic neutron absorption and maximize fuel content, such modules are relatively weak and tend to buckle under this column loading. The resulting distortion has been the cause of difficulties during refueling wherein a distorted module has interfered with removal of an adjacent module. Other potential difficulties include lack of straightness of control rod guide tubes and the resultant interference with control rod travel.
In designing a light water breeder reactor based upon PWR technology, the problem of module column loading has been exacerbated by special requirements of the breeder core design. The amount of structural material in the fuel assembly has had to be further reduced in order to enhance the neutron economy required to achieve breeding. The low water fraction necessitated by breeding considerations causes a high core flow resistance, thereby requiring higher compression hold-down spring forces to resist the resulting high upward pressure forces. Thus, with previous constructions, column loading increased while the structure designed to carry this loading became weaker.
In some nuclear reactors the core is supported by the pressure vessel head. Examples include the original core and present breeder core installed in the Shippingport reactor. These reactors have required removal of their control drive mechanisms (CDMs) for access to the modle support hardware and the disconnecting of all module supports prior to removal of the reactor vessel head. Relatively large pressure vessel head penetrations have been required to accommodate this support hardware.
In typical commercial PWRs the fuel modules are arranged in a parallel array, with relatively small pressure vessel head penetrations through which the CDM leadscrews pass. There are on the order of one hundred CDMs in such installations. The removal of so many CDMs for access to core suspension hardware is excessively time consuming, and the small leadscrew penetrations in the head could not accommodate this hardware. In this regard, it is noted that in a typical commercial PWR the CDMs remain installed on the pressure vessel head when the head is removed for refueling and the tops of the CDM pressure housings are removed for the access required to disconnect and remove the leadscrews prior to head removal.
As mentioned above and discussed in detail below, the present invention relates to a module support arrangement, and patents of possible interest in this regard include U.S. Pat. Nos. 3,163,585 (Metcalf et al); 3,604,746 (Notari); 3,857,599 (Jones et al); 3,905,634 (Johnson et al); 4,030,973 (Hoffmeister et al); 4,038,133 (Bitterman et al); and 4,279,699 (Kuhn). These patents relate to various forms of gripper, latching and support devices and arrangements for nuclear reactor components.